THE SECOND
 

The SI unit of time is the second, symbolized by the lowercase nonitalicized English letter s (or sometimes abbreviated as sec). It was defined originally as 1/60 of a minute, which is 1/60 of an hour, which in turn is 1/24 of a mean solar day. A second was thus thought of as 1/86,400 of a mean solar day, and this is still an excellent definition (Fig. 1). However, formally, these days, 1 s is defined as the amount of time taken for a certain cesium atom to oscillate through 9.192631770 x 10⁹ complete cycles.

Fig. 1. Originally, the second was defined as (1/60)(1/60)(1/24), or 1/86,400, of a mean solar day.

One second also happens to be the time it takes for a ray of light to travel 2.99792458 x 10⁸ m through space. This is about three-quarters of the way to the Moon. You may have heard of the Moon being a little more than one light-second away from Earth. If you are old enough to remember the conversations Earth-based personnel carried on with Apollo astronauts as the astronauts walked around on the Moon, you will recall the delay between comments or questions from earthlings and the replies from the moonwalkers.
The astronauts were not hesitating; it took more than 2 seconds for radio signals to make a round trip between Earth and the Moon. In a certain manner of thinking, time is a manifestation or expression of linear dimension, and vice versa. Both of these aspects of nature are intimately related by the speed of light, which Albert Einstein hypothesized is an absolute.